fn main() {
    println!("Hello, world!");
    let immutable_box = Box::new(5u32);
    println!("immutable_box contains {}", immutable_box);

    let mut mutable_box = immutable_box;
    println!("mutable_box contains {}", mutable_box);

    *mutable_box = 4;
    println!("mutable_bux now contains {}", mutable_box);
}
